What is the difference between NDVI, NDRE and MSAVI?

Normalised Difference Vegetation Index (NDVI) is a method of determining crop health by measuring the index of plant "greenness" or photosynthetic activity, and is one of the most commonly used vegetation indices.

NDVI is calculated on a per-pixel basis as the normalised difference between the red and near infrared bands of an image. Although NDVI is a great general indicator of crop health, it isn’t perfect for all crops, vigour or for all growth stages as it often saturates at the maximum value of 0.8.

This is where Normalised Difference Red Edge (NDRE) can provide a more accurate solution as it is able to detect variations in the health of cereal crops in their later stages, high biomass crops (including sugar cane), and permanent tree crops.

NDRE differs from NDVI by using the Red Edge (RE) part of the spectrum instead of Near Infrared (NIR). The Red Edge sensors are able to detect changes in chlorophyll content within the leaf and through the plant canopy, making it more effective in later crop stages where NDVI tends to become saturated.

The Red Edge Biomass will appear more pixelated than the NDVI due to the Red Edge being 20m resolution, whereas the Near Infrared is 10m resolution. Both layers are combined with the Red band, which is 10m resolution. The different layers are calculated using the following formulae:

NDVI = (NIR - Red) / (NIR + Red)

NDRE = (NIR - Red Edge) / (NIR + Red Edge)

Modified soil-adjusted vegetation index (MSAVI) aims at the other end of the growth cycle, where the crop has recently emerged and the canopy has not yet closed. Using NDVI often reads this as no vegetation and returns null results. MSAVI, on the other hand, corrects for the bare soil, adjusting the NDVI result to better reflect crop variation in young or sparse crops. MSAVI uses the formula:

MSAVI = (2 * NIR + 1 – sqrt ((2 * NIR + 1)2 – 8 * (NIR - Red))) / 2